Apparatus for the distillation of fresh water from sea water

ABSTRACT

In the distillation of fresh water from sea water, the sea water is passedownwardly in a falling film evaporator through a multiplicity of vertical tube evaporator (VTE) stages and multiple stage flash (MSF) evaporators. After the sea water passes through the first VTE stage where it forms a liquid component and a steam component, the liquid component is distributed evenly into the next VTE stage while the steam component enters an adjoining MSF stage. Condensed fresh water flows downwardly from one MSF stage to the next and experiences flash evaporation. The sea water or brine component and the fresh water distillate flow downwardly through the VTE and MSF stages so that the brine component can be removed from the final VTE stage and the fresh water distillate from the final MSF stage. The steam used to evaporate the sea water flows initially through the first VTE stage and then in combination with the steam component of the sea water through the next MSF stage and then continues alternately through the remaining VTE and MSF stages. Inert gases produced during the stagewise evaporation are removed periodically by suction. In the lower or low pressure VTE stages, for each VTE stage the condensed fresh water distillate is flashed in a plurality of MSF stages. The sea water is preheated before it is introduced into the first VTE stage and the primary steam used for heating the sea water and the steam component of the sea water are passed through the preheater for heating the sea water. The preheater and the VTE stages of the falling film evaporator are formed of stamped heat exchanger parts.

This is a divisional application of application Ser. No. 496,709, filedMay 20, 1983 and now abandoned.

SUMMARY OF THE INVENTION

The present invention is directed to a process of and apparatus fordistilling fresh water from sea water by film evaporation in amulti-stage vertical tube evaporation (VTE) process. In the VTE processthe sea water to be evaporated passes through tubular heat exchangersheated by primary vapor or steam.

As disclosed in German Pat. No. 23 34 481, sea water, heated in apreheater constructed as a stage heater, successively passes through theindividual stages of a falling film evaporator which is heated in thefirst stage only with primary steam and operates in the range of themaximum water boiling point. Partial evaporation of the sea water takesplace in the individual stages, while the unevaporated or liquidcomponent of the sea water flows into the next stage which is heatedwith a mixture of steam components. After passing through the finalstage, the sea water has been separated into brine and distillatecharacterized as clean or fresh water. The preheater and falling filmevaporator are separated into equal pressure and temperature stages, inaccordance with the boiling point reduction which can be obtained ineach stage and also in the form of tubular heat exchangers arranged asvertical columns within a support structure.

Such tubular evaporators are made up of densely packed tube bundlesarranged between flanges and the arrangement has a relatively highmechanical strength, particularly in the tube direction. As a result,the container enclosing the heat exchangers contains only the supportinggrates associated with the individual evaporating stages on which thetwo bundles are supported by means of their flanges. Such columns, whichgenerally have less than 15 stages, as a consequence, have an adequatemechanical strength so that there is no need to provide specialsupporting frames within the containers surrounding the tubular bundles.

Since tubular heat exchangers have an economic length of approximately 7m, the number of stages with a temperature difference of approximately+120° C. available for stage evaporation, though only 15 stages areemployed, have an overall height of the evaporation column is more than100 m. The efficiency in a multi-effect evaporation process, however, isdependent on the number of evaporation stages which can be obtainedwithin the pressure difference between the first and final stage of theevaporator available for sea water evaporation.

Consequently, there is a need to increase significantly the number ofstages in such columns, but such an increase can not be obtained, for anumber of reasons, with tubular exchangers. This problem can be overcomeby the use of stamped exchanger plates which have reinforcingprojections and depressions arranged in a uniform manner and in agrid-like form in the two directions of the plates. The plates arearranged in pairs in a mirror-symmetrical manner so that theprojections-depressions oriented in one direction form tube-like ductsand those oriented in the other direction define slot-like ducts on theopposite sides of the plates. Accordingly, depending on whether theexchanger plates are used in a preheater or a falling film evaporator,the steam flows through either the tube-like ducts or the slot-likeducts and the sea water flows through either the slot-like ducts or thetube-like ducts. Such identical heat exchanger plates form a type offiller within a vertical column, and, as a result, are required in largenumbers, especially if more than 50 stages are required for performingthe multi-effect evaporation process. Accordingly, gratings held by thecontainer wall enclosing the heat cxchanger plates are no longersufficient for a predetermined arrangement of such a large number ofsuch plates within a pressure container for maintaining the desiredarrangement of the individual heat exchanger plates. Therefore, a needis present for a support structure for a pressure-tight container whichis particularly suitable for this purpose. Furthermore, whendesalinating sea water by the multi-effect evaporation process, it isunavoidable that gases, particularly inert gases, collect in theindividual stages of the falling film evaporator and such gases must beremoved by suction so as not to impair the condensation process. Thisrequirement causes considerable problems in the case of tubularexchangers, because there is either a precise condensation end of thestage, nor any possibility of suction removal at such an end, due to thespatial arrangement of the individual tubes in bundles.

Moreover, it is difficult in the case of tubular exchangers todistribute uniformly the sea water to be evaporated onto the surfaces ofthe inner walls of the tubes held by the flanges in each stage. Onlywith the uniform distribution of the sea water at the beginning or inletof each stage is it possible to obtain the desired homogeneous liquidfilms, flowing on the inner circumferential surfaces of the tubes, andwhich are necessary for effective evaporation. Such liquid films,however, cannot be maintained over tube lengths of approximately 7 m.

Finally, considerable difficulties are encountered in connection withthe assembly and maintenance of such columns constructed as tubularexchangers. As residues are unavoidable in evaporation processes forproducing fresh water from sea water, maintenance costs, apart fromenergy costs, considerably influence the balance of costs.

In a multiple stage flash evaporation (MSF) process the heated sea waterto be evaporated flows through numerous flash chambers with weirs andpassages in the lower parts of such chambers. Preheater tubes arelocated in the upper parts of the flash chambers through which theincoming sea water is heated and on which the steam generated from thesea water is condensed. The fresh water distillate and the brineconcentrate can be removed from the final flash chamber, note Ullmann,"Enzyklopadie der Chemie", 3rd edition, vol 18, p. 465.

The efficiency in such an arrangement is dependent on the number ofevaporation stages which can be obtained within the temperaturedifference between the first and final stages available for theevaporation of the sea water. Admittedly, the horizontally orientedcellular construction in MSF process plants permits a larger number ofstages than in VTE process plants, up to the present time 36 stages havebeen obtained, however, in flash evaporation more than 60% of the totalcosts are involved in the actual evaporation of the sea water, so thatsuch plants have not been successfully comparable with so-called fallingfilm evaporators, which have a significantly higher evaporationefficiency. In addition, plants operating on the basis of the MSFprocess require considerably more space than the vertically constructedplants for falling film evaporators of the VTE process.

The primary object of the present invention is to provide a novelprocess and apparatus for distilling fresh water from sea water so thatthe fresh water yield per energy unit consumed is improved in asignificant manner. Further, another object is to provide a supportingframe for the evaporation column located within a pressure-tightcontainer, particularly for desalinating sea water by the multi-effectevaporation process. The supporting frame not only fulfills thenecessary supporting function, but simultaneously is constructed so thata stage separation and a permanent maintenance of the geometricalarrangement of the individual and identical heat exchanger plates ispossible, without special precautions or additional components requiredon the heat exchanger plates.

On the basis of a process for the distillation of fresh water from seawater by film evaporation in a multiple stage VTE process, in accordancewith the present invention, after the sea water passes through the firstVTE process stage and before the liquid component of the sea waterenters the following VTE stages, the liquid component is distributedover a weir-like member and condensed fresh water passing into at leastone MSF process stage associated with an individual VTE stage undergoesflash evaporation as it passes from one MSF stage to the next. The freshwater distillate and the increasingly concentrated liquid sea watercomponent or brine are passed downwardly through the following VTEprocess and MSF process stages so that the brine concentrate can beremoved from the final VTE process stage and the fresh water distillatefrom the final MSF stage. The steam for heating and evaporating the seawater is successively passed through all of the evaporation stagesstarting with the first VTE process stage and subsequently is mixed withthe steam components from the evaporated sea water from the subsequentVTE process stages. The inert gases separated during the stagewiseevaporation are removed by suction.

In accordance with the present invention, an apparatus for carrying outthe multi-effect evaporation process utilizes heat exchanger plates forthe preheater and for the falling film evaporator. The arrangement ofheat exchanger plates is located within and between the facing sidewalls of a pressure-tight container located within a supporting frame.Bearing surfaces are provided within the container corresponding to thenumber of slide-in units in the stages of the falling film evaporator.The preheater is positioned within the container extending over theheight of the VTE stages. Deflectors are provided within the containerfor each of the pressure stages, corresponding to the number of slide-inunits, so that each of the stages is provided in a pressure-tight mannerwith each stage of the falling film evaporator corresponding to asimilar pressure stage of the preheater.

Further features of the invention are set forth in the claims.

By the combination, in accordance with the present invention, of the twoknown evaporation processes to form a single evaporation process, it ispossible to utilize not only the evaporation energy supplied in anoptimum manner, but it is also possible to provide an extremely simpleconstruction of the evaporation units if stamped, flat heat exchangerplates are used both for the preheater and the falling film evaporatorpermitting the establishment of a large random number of stages forutilizing an available temperature and pressure gradient and a simpleconstruction in a common vertically extending pressure-tight container.The MSF process stages interposed between the VTE stages and thepreheater stages can be formed as curved deflectors or baffles whichsimultaneously serve as the defining boundaries for the pressurechambers of different pressure levels. Accordingly, the verticallyarranged preheater housed in the same pressure-type container, has itsheating surfaces connected directly to the steam components in theindividual VTE and MSF process stages.

In the uppermost or first VTE process stage, the sea water, heated toapproximately 130° C. in the preheater, is evaporated by indirectcontact with a supply of primary steam at 130° C. The primary steamcondensate from the first VTE stage is introduced into the distillationprocess in the first MSF process stage which is in direct flowcommunication with the first VTE stage. The fresh water distillatecollected in the MSF stages flows downwardly to the last or lowermoststage and then passes out of the pressure-tight container at atemperature of approximately 28.9° C. The liquid sea water component, orbrine, flowing from the outlet of the first VTE process stage is dammedup by a weir-like member at the inlet to the next VTE process stage. Theweir-like or distribution member provides a liquid level at the entranceto the VTE process stage. Due to an overflow and a restrictor, a part ofthe liquid sea water component or brine passes directly into theadjoining MSF stage which has a lower pressure than the preceding stageso that flash evaporation takes place. The main portion of the liquidsea water component or brine flows about the distributor member into theVTE process stage. As a result of the pressure reduction and heattransfer through the condensing distillate steam from the precedingprocess stage, a portion of the brine flowing through the VTE stageevaporates. At the outlet from the VTE process stage the vaporized seawater separates from the liquid component of the sea water and flowsinto the corresponding MSF stage while the sea water liquid componentagain collects at the inlet of the following VTE process stage forcontinuing the evaporation process. These process steps are repeated atmodified saturated steam temperatures and pressures until all of the VTEprocess stages have been traversed by the sea water liquid component orbrine which has an increasing salt concentration. The steam generated inthe falling film evaporator is passed through deflectors located at theopposite ends of the individual VTE stages and the deflectors act ascentrifugal drip separators for the liquid and steam components of theflow entering the corresponding MSF stage. The steam generated in theVTE stage is condensed on the heat exchanger surfaces in thecorresponding pressure stage of the preheater. The condensate is alsocollected in the deflectors and is supplied via restrictors to the nextlower pressure level of the MSF stages in the column so that at least apart of the fresh water distillate is flashed as it enters the followingMSF stage whereby it flows through the corresponding VTE stage fortransferring heat to and evaporating the down-flowing liquid componentof sea water.

In a multi-effect evaporator process where there are more than 43process stages in the operating column, in the upper 43 process stagesthe steam component from each VTE process stage is directly mixed withthe components in the corresponding MSF stage as described above. Ifthere are a total of 55 VTE process stages in the operating column, inthe last or lower 12 process stages, the fresh water distillate in eachstage is flashed in a plurality of corresponding MSF stages. The steamproduced in these lower MSF process stages is condensed only on thetubular heat exchanger surfaces of the corresponding stages of thepreheater.

The condensates collecting in the deflectors or baffles constructed ascentrifugal drip separators is supplied to the following process stageby restrictors restricted in accordance with the pressure differences.The individual stages are sealed from one another for maintaining thepressure differences. The inert gases separated out within the columnfor each flash process and condensation are removed by suction at twopoints from each process stage with the suctioning action taking placeat the end points in the condensation process. Inert gases obtained inthe VTE process stages are removed by suction to a cavity between thedeflectors and the wall of the container along, in each case, thelowermost gap cross-section in each VTE process stage, and specificallyin each stage at the center between the opposite ends of the heatexchanger surfaces. The inert gases contained in the condensationprocess in the preheater are also removed by suction throughcorresponding openings in a wall of the container via a suitable cavitybetween the preheater and the wall at the condensation end point. Theinert gas suction capacities for the individual stages are, in eachcase, regulated by throttle valves in the individual connecting lines.

The process according to the invention makes it possible to more thandouble the efficiency and, as a result, the output as compared withknown processes.

In the present invention, the pressure-tight container affords asupporting frame for the evaporation column and a particularly simpleand cost-saving construction is obtained, without any reduction inoperational reliability. The container is constructed of fixed andmovable side walls with corresponding deflectors for forming theindividual stages so that the installation and maintenance of the columnis facilitated to a considerable extent. After removing the movable sidewalls of the container, it is merely necessary to detach one deflectorstage so that a slide-in unit forming one of the stages can be removed.Accordingly, the time-consuming and costly detachment of connectionsexperienced in the past is rendered completely superfluous.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there are illustrated and described preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings:

FIG. 1 is a process diagram of the sea water desalination apparatusembodying the present invention illustrating certain of the initialstages, several of the intermediate stages and several of the finalstages of the apparatus;

FIG. 2 is a perspective view of a part of the sea water preheater asshown in FIG. 1;

FIG. 3 is a perspective view of a portion of the falling film evaporatoras shown schematically in FIG. 1;

FIG. 4 is perspective diagrammatic view of a portion of the falling filmevaporator and preheater of the present invention illustrating the firstand last stages of the apparatus;

FIG. 5 is a sectional view on an enlarged scale showing a portion of thefalling film evaporator and the preheater illustrated in FIG. 4illustrating two VTE process stages and one or more MSF stages; and

FIG. 6 is a partial perspective view of one stage of the falling filmevaporator shown in FIG. 3, however, on a much larger scale;

DETAIL DESCRIPTION OF THE INVENTION

As shown in the process diagram of FIG. 1, cold sea water is supplied bya pump 10 into the lower end of a vertical sea water preheater VW madeup of a number of stages, that is stages VSt_(n) to VSt₁, where the seawater is heated with condensing steam. After its upward path through thepreheater, the heated sea water is evaporated in a vertical falling filmevaporator FV also made up of a plurality of stages, that is, stagesFSt₁ to FSt_(n) with the sea water being evaporated by condensing steam.Initially, primary steam is introduced through a pipe 11 to the firststage FSt₁ which forms the first vertical tube evaporation (VTE) processstage and, at the same time, the primary steam serves to heat theuppermost stage VSt₁ of the preheater VW. The preheated sea water passesover an overflow 12 into the inlet of the first stage FSt₁ of thefalling film evaporator FV and as it flows downwardly through thesuccessive stages the sea water is divided into a liquid component and asteam component. The liquid component passes into the inlet of theadjacent next lower stage of the falling film evaporator FV and so onuntil it passes through each of the stages FSt₁ to FSt_(n). The steamgenerated within the first VTE stage and the steam developed fromflashing the condensate passing downwardly from the first stage of themultiple stage flash (MSF) evaporator is used in heating the secondstage FSt₂ of the falling film evaporator. The steam component is alsoused to heat the corresponding stages VSt₂ to VSt_(n) of the preheater.In the vertical columns formed by the falling film evaporator and thepreheater, the stages of each are generally paired as shown in FIG. 1 sothat the process, to be described in greater detail below, is repeatedin each stage. The steam remaining in the final stage VSt_(n) iscondensed and is evacuated together with the remaining fresh waterdistillate passing downwardly through the MFS stages by the pump 23.After the salt water concentrate or brine flows through all of the VTEstages, it is possible, while the fresh water is removed by the pump 23,that the sea water concentrate is collected below the final stageFSt_(n) at the collector 25 and is carried off through the pump 24. Eachof the interconnections between the VTE stages and the preheater stages(VN) forms at least one MSF stage.

The sea water falling film evaporator FV is made up of a multiplicity ofidentical pairwise-joined heat exchanger plates 30 which will now bedescribed with reference to FIGS. 3-6.

Each heat exchanger plate 30 is a rectangular stamped sheet with agrid-like arrangement of reinforcing projections 32, 33 formed in thelongitudinal and transverse directions of the plate, note FIG. 6. Whilethe projections 32, 33 extend from opposite sides of the plate, theseprojections on one side of the plate form corresponding recesses ordepressions in the opposite side. Grooves 32' are provided betweenadjacent long sides of the projections 32. The heat exchanger plate arestacked alongside one another in pairs so that the projections in oneplate extend in a line offset from the projections in the other plateand the plates are welded together along the side edges 40', note FIG.6, that is the edges extending parallel with the shorter sides of theprojections. Accordingly, this pair of plates defines a number ofslot-like ducts 35 extending between the projections 33. If two suchplate pairs are stacked one against the other with the projections 32 inopposite alignment, that is in a mirror-symmetrical manner, tubularducts are formed between the contacting projections of each adjacentpair of plates. Note the arrangement of the plates in FIG. 6.Accordingly, in one direction a plurality of juxtaposed tubular ducts 34extend at right angles to a plurality of juxtaposed slot-like ducts 35,so that the heat exchanging media pass in cross-flow to one another. Asviewed in FIG. 6 the tubular ducts 34 extend vertically while theslot-like ducts extend horizontally. The adjacent edges 40' of the twoplate pairs, note the upper end in FIG. 6, define slots 42 extendingacross the plates transversely of the longer dimension of theprojections 32. In the case of the falling film evaporator the slots 42are located at the inlet and outlet ends of the downwardly flowing seawater. In each of these slots at the inlet side, a rod 41 is placed sothat it extends across the full length of the inlet. The rods 41 act asdistributors and also as weir-like members for the sea water passingdownwardly from one stage of the falling film evaporator to the nextdownwardly located stage. As a result, steam pressure between theindividual tubular ducts of the adjacent stages and the differences inthe salt concentration of the sea water are compensated. In eachinstance, the slots are disposed opposite the outlet end of the adjacentupwardly located stage of the falling film evaporator. The stacked heatexchanger plate pairs are held in position by side walls 205, 206 of thecontainer, note FIG. 3. Thus, the tubular ducts 34 formed by theadjacent pairs of heat exchanger plates 30 and extending in the longerdirection of the projections 32 form the evaporation surface for the seawater flowing through each stage of the falling film evaporator, whileon the slot side formed between the other plates, the steam required toevaporate the sea water flows. The projections 33 of the heat exchangerplates form cross connections for the sea water flowing through thetubular ducts 34. The liquid component-steam component mixture leavingan individual stage of the tubular ducts 34, as a result of the crossconnection formed by the projections 33, is distributed uniformly intothe tubular ducts in the following stage so that the pressure within theindividual tubular ducts as well as the differences in the saltconcentration entering the ducts is compensated before the flowcommences downwardly through each stage due to the distributing effectof the rods 41.

The sea water preheater VW, shown in part in FIG. 2, is constructed ofthe same heat exchanger plates 30 combined in the same manner of pairsof plates. In the preheater VW, as compared to the falling filmevaporator FV, the orientation of the projections 32 and 33 are rotatedthrough 90° so that the tubular ducts 34 extend horizontally while theslot-like ducts extend vertically.

As illustrated in FIG. 4, the sea water preheater VW extends for thefull vertical height of the column defined by the multi-effect unit andis subdivided by transversely extending unstamped areas 37 within theindividual heat exchanger plates 30 into a number of sectionscorresponding to the desired number of preheater stages. Thissubdivision of the preheater, while determining the pressure of the seawater flowing upwardly through the slot-like ducts, forms a separationbetween the individual stages of the preheater containing the tubularducts 34. Because of the unstamped areas 37, the surfaces of the platesare placed against one another and define the separation betweenadjacent preheater stages. As a consequence, the number of tubular ducts34 in each stage of the preheater is determined by the spacing betweenthe adjacent unstamped areas.

The arrangement of the unstamped areas 37 is, in each case, determinedin accordance with the pressure and temperature stages of themulti-effect column selected for the evaporation process as a functionof the temperature difference. In the illustrated embodiment, 55 stagesare provided between the inlet stage and the outlet stage, and thedistance between two adjacent unstamped areas 37 is smaller, stage tostage, from the inlet to the outlet, note FIG. 4, and, as alreadyindicated, the number of tubular ducts 34 varies from stage to stage,decreasing toward the upper outlet end of the preheater.

The width of the slot-like ducts 35 is determined by the spacing betweenthe projections 33 stamped in the plates, that is, when the pairs ofexchanger plates are stacked alongside one another so that there is aplurality of separated slots in the pairs of plates forming the slotside of the heat exchanger. If the stamping operation forming theprojections is not carried out at a particular location, such aslocation 39 in FIG. 3, there is a bypass formed between adjacentslot-like ducts 35.

The cold sea water is introduced into the slot side of the first bottomstage of the preheater and passes upwardly until it reaches the finaltop stage. While the sea water to be preheated passes verticallyupwardly the heating steam is passed in a stagewise manner at rightangles through the tubular ducts formed in each of the individualpreheater stages. In the final uppermost stage of the preheater, onlyhot primary steam is passed through the tubular ducts while in all ofthe remaining stages a secondary steam flows through the tubular ducts,that is, a steam mixture provided by the combination of the primarysteam and the steam generated in the stages of the falling filmevaporator which will be described later. Accordingly, the lowest steamtemperature flows through the inlet stage of the preheater correspondingto the lowest temperature of the sea water entering the initialpreheater stage.

In FIGS. 3 and 4 the stagewise arrangement of the heat arrangement ofthe heat exchanger plate pairs with the falling film evaporator FV isare illustrated. While the vertically arranged sea water preheater VWextends over the full height of the multi-effect column as an assemblysubdivided into stages by the unstamped areas 37, falling filmevaporator FV is subdivided into individual assemblies or sectionscorresponding to the number of stages of the column extending in anumber of planes E₁ to E_(n). The number of planes E₁ to E_(n)correspond to the number of preheater stages so that for each plane withthe desired pressure and temperature stages St₁ to St_(n) for themulti-effect evaporation process there is a corresponding stage of thefalling film evaporator and of the preheater. Further, for each combinedfalling film evaporator-preheater stage there is at least one separateMSF evaporator stage, note FIG. 1.

For the subdivision of the container BE into the individual stages aframe construction is provided within the container made up of the fixedside walls 205, 206 and the movable side walls 207, 208. The individuallevels of the stages of the falling film evaporator are formed by theshaped members 301 extending transversely between the fixed side walls205, 206. The spacing between the individual stages of the preheater isdetermined by the individual shaped members 309 mounted on the movableside wall 208. Accordingly, the number of planes E₁ to E_(n) have aspacing determined by the spacing between the individual frame members.On each level of the stage members 301, a group of the heat exchangerplates 30 are arranged forming a stage of the falling film evaporator.Each stage of the falling film evaporator has a corresponding stage inthe preheater. As shown in FIG. 3, deflectors or baffles LB are providedin each combined stage for affording flash evaporation so that steam,the sea water liquid component or brine and fresh water is supplied toeach of the individual combined stages. Similarly, with regard to thediagrammatically represented suction system, note FIG. 1, thenon-condensable inert gas produced in the condensation zone in eachstage of the falling film evaporator, can be removed. In each stage ofthe falling film evaporator, the condensation zone is formed by anunstamped area 39 of the heat exchanger plates 30 of the falling filmevaporator, that is, the center of the plates extending in the directionfrom the deflectors LB to the opposite side wall 207, note FIG. 3. Thisunstamped area 39 interconnects all of the slot-side ducts 35 of theassembled pairs of plates in the stage. Therefore, the inert gascomponents can be removed at the bottom slot of the heat exchanger platepairs for each stage of the falling film evaporator.

As has already been mentioned, the steam pressure within the individualtubular ducts 34 of the falling film evaporator is compensated by therods 41 extending across the inlet ends of the plates forming thetubular ducts 34. Similarly, in addition to balancing the pressure, therods also provide a balancing effect of the salt concentration of theliquid component of the sea water flowing downwardly through the tubularducts. The rods 41 correspond to the length of the inlets into eachstage of the falling film evaporator and the rods are located withinslots 42. The slots 42 are formed by the unstamped ends of the plates,note FIG. 3, which unstamped ends are welded together by means of a rollseam weld joining two heat exchanger plates 30 so that the adjoiningplates of the pairs of plates joined together by such weld form theslots 42 and also form between them the tubular ducts 34. The rods 41provide a weir-like arrangement for the downwardly flowing liquidcomponent of the sea water exiting from the outlet of the adjacentupwardly located VTE stage.

All of the sea water preheater and VTE and MSF evaporator stagessupported by the frame construction are laterally enclosed by apressure-tight container BE shown in part and, in turn, held by asupport structure SG. The evaporator stages are connected to the vacuumsystem.

In the embodiment illustrated in the drawings, particularly in FIG. 1,the combined multi-effect evaporation process for sea water desalinationusing the stamped heat exchanger plates 30 are divided into 55 processstages for the sea water preheater VW and for the falling filmevaporator FV forming the VTE process. In addition, 91 MSF stages,defined by the deflectors LB, note FIG. 3, extending across the width ofeach stage are located between each of the individual VTE stages. Asingle MSF stage is provided for each of the first 43 VTE stages, while4 MSF stages are provided for each of the last 12 VTE stages, note inparticular FIGS. 1 and 5.

In the first 43 VTE process stages of the falling film evaporator FV,that is starting from the upper end of the falling film evaporator,deflectors LB are provided for each stage. For each stage, otherdeflectors 306 and 310 are provided on the side of the stage remote fromthe preheater VW and adjacent the slidable wall 207. The deflectors LBare formed by deflectors 304, 305 and 308 are located on the oppositeside of each stage adjacent to the sea water preheater VW. Deflectors306 and 310 are constructed as centrifugal drip separators and arecurved in the form of an open ellipse, while deflectors 304 are curvedin the manner of an angle with a short side and a long side.Accordingly, the combination of the walls of the deflectors 305 and thedeflectors 304 define U-shaped ducts 312 a in which condensed freshwater is collected from the preceding VTE stage. A series of holes 313are provided in the bottom of the ducts 312 so that the condensed freshwater can flow downwardly from the U-shaped ducts into the nextsubjacent MSF stage where at least some of the fresh water is flashedinto steam. As can be seen in FIG. 1, for each of the bottom 12 VTEstages there are four deflectors 305 for each deflector 304, that is,corresponding to the stages FSt₄₄ to FSt₅₅ in FIG. 1, and 7, so thatfour MSF process stages are associated with each of these VTE processstages.

The upper ends 314 of the deflectors 304 as well as the deflectors 306,307 and 308 form sealing surfaces for the individual falling filmevaporator stages FSt and the heat exchanger plates forming each of thestages are constructed as slide-in units. To ensure reliable sealing,silicone seals 315 are associated with each of these sealing members.Further, as can be seen in FIG. 5, the angular deflectors 310, fixed tothe movable container wall 207, are engaged at their lower U-shaped ends316 through a sealing member 315 to a part 317 of the deflector 306. Anadditional sealing member 315 is also provided between the deflector 306and the heat exchanger plates. The deflectors secured in a fixed mannerbetween the fixed side walls 205 and 206 form tie rods for the fixedcontainer walls, particularly in the region of the container BE wherethe preheater is located. The deflectors 305 with their end partsforming the sealing surfaces 318 also have interposed sealing members315 which engage on the facing surfaces of the corresponding stages VStof the preheater VW and, in particular, at the locations where theunstamped areas 37 subdivide the preheater into its individual stages.Further, deflectors 309 are provided between the movable side wall 208of the container BE and the adjacent sides of the preheater VW. Sealmembers 315 are provided between the sealing surfaces formed by theportions 319 of the deflectors 309 and the adjacent preheater.

Therefore, by means of the deflectors 304 to 310, the individual stagesof the multi-effect apparatus are interconnected in a pressure-tightmanner. The steam generated by each stage of the VTE/MSF evaporator canflow to the associated stage of the preheater where it serves to heatthe upwardly flowing sea water.

In the embodiment illustrated in the drawings, the individualreinforcing projections 32 of each heat exchanger plate 30 has a lengthor long dimension of 35 mm, and the tubular ducts 34 of the sea waterpreheater have a length of 350 mm. The slot-like ducts of the fallingfilm evaporator have a long dimension extending transversely of thecolumn of 2160 mm, and the individual stack of plates forming a fallingfilm evaporator sliding unit or stage have a thickness of 500 mm and theoverall column has a height of 34000 mm.

The apparatus, as described above, functions in the following manner. Asindicated above, all of VTE and MSF process stages, with the exceptionof the final condenser, are housed in the pressure-tight container BE.The various pressure chambers are defined directly by the individualstages of the falling film evaporator and the connected deflectors orbaffles LB of the MSF stages. Preheater VW is vertically incorporatedinto the container and its heating surfaces are connected directly withthe individual steam chambers of the VTE and MSF stages.

In the initial VTE process stage FSt₁ of the falling film evaporator FV,the sea water heated in the preheater VW to 130° C. is evaporated at130° C. by means of the primary steam introduced through the line 11.The primary steam condensed within the first VTE stage is supplied bymeans of the first MSF stage and with the condensed fresh water ordistillate component produced in the process passes through all of the91 MSF stages and exits from the container along with the condensedfresh water obtained from the sea water, at a temperature ofapproximately 28.9° C.

After flowing through the first stage, the liquid component of the seawater or brine is held at the inlet of the next VTE stage by the rod orweir-like distributor 41 which forms a seal-like member between theadjacent stages and also provides a distributing effect for introducingthe liquid component of the sea water into the next VTE stage of thefalling film evaporator. As a result of the rod 41, there is an overflowof a small amount of the liquid component or brine which is introduceddirectly into the next MSF stage which is at a lower pressure and flashevaporation occurs. The main volume of the liquid component of the seawater passes into the next subjacent VTE stage. Through the pressurereduction and the heat transfer from the condensing fresh water steam ofthe preceding stage, a portion of the liquid volume flowing downwardlyto the tube ducts evaporates in each stage of the falling filmevaporator heat exchanger. After passing out of each stage of thefalling film evaporator, the evaporated sea water or steam and theliquid component of the sea water are separated. The remaining liquidcomponent or brine follows the same process in each of the following VTEstages each time at modified saturated steam temperatures and pressures.

The distillate or steam is passed by the deflectors 304 which also actas centrifugal drip separators, to the condensation surfaces of the nextfollowing stage where the steam is condensed in the falling filmevaporators of the VTE stage and also on the heat exchanger surfaces ofthe preheater. The resulting condensed fresh water is collected by thedeflectors 305--ducts 312, note FIG. 5, and passes through therestrictors or holes 313, to the next lower pressure level where againat least a part of the condensed fresh water is flash evaporated.

In the upper 43 stages, the flash evaporated steam is directly mixed andcondensed with the steam component of the particular VTE stage in themanner described above. In the lower 12 stages, however, the fresh watercondensate is not flash evaporated in one stage, rather the pressuredifference of the particular VTE process stage is subdivided into aplurality of flash stages, for example, four such stages. The steamproduced by these MSF stages is condensed only on the tube-side heatexchanger surfaces of the preheater VW.

The salt water collecting in the centrifugal separators is suppliedagain to the following stage through restrictors based on the pressuredifferences.

The inert gases obtained during each flash evporation and condensationare drawn out of each stage at two points, suction takes place at theend points of the condensation process. The inert gases obtained in theVTE stages is removed by suction through a cavity 400 located betweenthe deflectors and the movable side wall 207 of container BE and also atthe lowermost slot cross-section 410 of the falling film evaporator,located at the center or unstamped area 39 of each heat exchanger platessurface. The inert gases obtained during the condensation process in thepreheater are removed by connections 403 at the other movable side wall208 of the container BE via a cavity 404 between the preheater and theside wall at the condensation end point. The inert gas suction capacityfor the individual stages is controlled by throttle valves 405 inconnecting lines 406.

The described combination of VTE and MSF stages leads to numerousadvantages which provide for the production of fresh water in amulti-effect procedure which has not been considered possible in thepast. The particular contribution to this multieffect procedure isafforded by the use of the known plate-type heat exchanger. For example,it is immediately apparent that, as a result of the limited free "tubelength" of only 35 mm for the individual falling film evaporatorelements of the stages, as provided by the stamped projections 32, afilm is maintained over the entire length of the tubular ducts 34 which,as mentioned above, is not possible in falling film tubular evaporatorswhere the tube length is up to 7 m. The use of the described heatexchanger elements also makes it more possible, than has been the casein the past, to subdivide the available temperature and pressure rangeinto a random number of individiual stages and this characteristic orfeature has a very favorable influence on the efficiency of themulti-effect column.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles.

I claim:
 1. Apparatus for the distillation of fresh water from seat water comprising a generally vertically extending falling film evaporator comprising a vertical tube evaporator formed of a multiplicity of vertical tube evaporator stages with said evaporator stages spaced apart in the vertical direction and including an uppermost stage, a lowermost stage and a multiplicity of vertically spaced stages therebetween, means for supplying preheated seat water to the uppermost stage of said vertical tube evaporator so that the seat water flows downwardly through said stages, each of said vertical tube evaporator stages of said vertical tube evaporator comprises upwardly extending stamped heat exchanger plates having opposite vertically extending planes with shaped reinforcing projections stamped out of the opposite planes of said plates with the projections forming shaped recesses on the opposite sides of said plates from said reinforcing projections, said projections and recesses arranged in a grid-like manner in both directions of said plates, a first pair of said plates arranged with said projections aligned in opposed relation with said projections extending in the opposite direction combining to form tubes extending in the vertical direction of said plates, the recesses in said plates aligned vertically with said projections forming said tubes defining cross connections between said tubes for providing a pressure balancing effect, another said plate on each side of said first pair with said projections of the another said plate offset transversely of the vertical direction relative to said projections in said first pair and the another said plate and the adjacent one of said plates of said first pair defining therebetween slot-like ducts extending transversely of said tubes, in said vertical tube evaporator said tubes are arranged for the downward flow therethrough of the preheated seat water and said slot-like ducts are arranged for the flow therethrough of a steam component for vaporizing the sea water flowing through said vertical tubes so that the flow through said vertical tubes is generally horizontal, and the upper ends of said plates forming said tubes are spaced apart forming an inlet to each of said vertical tube evaporator stages, a loosely arranged horizontally extending distributor tube located in each said inlet extending for the horizontal length of said inlet across the upper ends of said plates and said rod forming a weir-like member for uniformly distributing the downwardly flowing seat water into said tubular ducts in each said vertical tube evaporator stage.
 2. Apparatus, as set forth in claim 1, wherein said projections in said heat exchanger plates having a first longer dimension and a second shorter dimension, one group of said projections forming said tubular ducts in said falling film evaporator have a first dimension thereof extending vertically and the other said projections having the first dimension thereof extending approximately perpendicularly to the one group of projections, and the other group of projections forming cross connections for said tubular ducts.
 3. Apparatus, as set forth in claim 1, wherein the number of heat exchanger plates in each VTE stage of said falling film evaporator differs so that by reducing the number of said heat exchanger plates and increasing the spacing between the VTE stages, the flow cross-section for the steam component is increased in the stages starting from the uppermost stage where the preheated sea water is introduced into the upper end of said vertical tube evaporator extending to the lowermost stage at the lower end of said vertical tube evaporator.
 4. Apparatus, as set forth in claim 1, wherein said heat exchanger plates forming said slot-like ducts of said vertical tube evaporator have upwardly extending unstamped areas centered between the ends of said slot-like ducts so that the inert gas collected in said slot-like ducts can be removed from the lower portion of the space formed between the unstamped sections.
 5. Apparatus, as set forth in claim 1, comprising a pressure-tight container laterally enclosing a vertically extending multi-stage preheater and said vertical tube evaporator, with said preheater and vertical tube evaporator having corresponding stages, said container having two pairs of facing side walls forming a rectangular horizontal cross-section, means within said side walls for forming supporting surfaces for each of said vertical tube evaporator stages so that each stage is formed as a slide-in unit, means for supporting said preheater within said container, and deflectors for directing flow between corresponding stages of said vertical tube evaporator extending between a pair of said side walls between each of said vertical tube evaporator stages and the corresponding stage of said preheater, and means within said container for separating the adjacent stages of said slide-in units and said preheater in a pressure-tight manner.
 6. Apparatus, as set forth in claim 5, wherein each of said slide-in units of said vertical tube evaporator stages are formed of a plurality of heat exchanger plates, and said preheater extends continuously for the height of said vertical tube evaporator stage.
 7. Apparatus, as set forth in claim 5, wherein each of corresponding stages of said vertical tube evaporator and said preheater are interconnected by said deflectors, said deflectors are secured in part to one pair of said side walls of said containers and in part to the other pair of said side walls of said container.
 8. Apparatus, as set forth in claim 6, wherein a first one of said side walls extending along one side of said preheater, a second one of said side walls located opposite said first one of said side walls and extending along one side of said vertical tube evaporation stages, deflectors extending between said one of said side walls and said preheater and deflectors extending between said second one of said side walls and the adjacent sides of said vertical tube evaporation stages, and the first one and second one of said side walls being removable from said container and said deflectors being welded to said first one and second one of said side walls.
 9. Apparatus, as set forth in claim 5, wherein deflectors are located between one end of said vertical tube evaporator stages defining the ends of said slot-like ducts and extending to the adjacent side of said preheater, and said deflectors are formed as fresh water collectors.
 10. Apparatus, as set forth in claim 8, wherein said deflectors extending between the second one of said side walls and the adjacent ends of said vertical tube evaporator stages have a U-shaped configuration and serve as tie rods for said container.
 11. Apparatus, as set forth in claim 9, wherein said deflectors located between the adjacent sides of said vertical tube evaporator stages and said preheater stages form tie rods for said container.
 12. Apparatus, as set forth in claim 5, wherein said deflectors include deflectors associated with the upper end of each said vertical tube evaporator stage and said deflectors are detachably connected to said side walls located at the opposite ends of said deflectors.
 13. Apparatus, as set forth in claim 5, wherein sealing means are arranged between said deflectors and the adjacent ones of said vertical tube evaporator stages and said preheater.
 14. Apparatus for the distillation of fresh water from sea water comprising a generally vertically extending preheated divided into a number of vertically spaced preheater stages including an uppermost stage, a lowermost stage and a plurality of vertically spaced stages therebetween, each of said preheater stages comprises continuous upwardly extending stamped heat exchanger plates having opposite vertically extending planes with shaped reinforcing projections stamped out of the opposite planes of said plates and said projections forming shaped recesses on the opposite sides of said plates from said reinforcing projections, said projections and recesses arranged in a grid-like manner in both directions of said plates. a first pair of said plates arranged with said projections aligned in opposed relation with said projections extending in the opposite direction combining to form tubes extending in the horizontal direction of said plates, another aid plate on each side of said first pair with said projections of the another said plate offset relative to said projections in said first pair and the another said plate and the adjacent one of said plates of said first pair defining therebetween slot-like ducts extending transversely of said tubes in the vertical direction, said slot-like ducts are arranged to receive sea water for the upward flow therethrough of the sea water from the lowermost stage to the uppermost stage, and said heat exchanger plates forming said preheater having horizontally extending unstamped areas spaced apart in the vertical direction and dividing said preheater into individual stages, and said unstamped areas effecting a balance of the pressure of the sea water flowing upwardly through the slot-like ducts, and the location of the unstamped areaa defining the space between adjacent stages of said preheater and the number of said unstamped areas defining the number of the stages of said preheater.
 15. Apparatus, as set forth in claim 14, wherein the heating surface required for each stage of said preheater is determined by selecting the number of tubes of said heat exchanger plates located between the unstamped areas spaced apart in the vertical direction of said preheater. 